4.x Users can Upgrade to this latest version for Free, while 5.0 Users can Update to 5.1:

New Features in Version 5.1
Substantial improvements in performance and speed
CPU Resource Control allows user command of CPU usage
ACPI support increases compatibility with imported or exported PC disk images using Technologies such as Norton Ghost.
Increased RAM Support - user can assign up to 1 GB RAM per virtual machine.

More Info:
_http://www.connectix.com/support/vpcw_online.html